Sensitometry



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J. o. BAKER 2,361,447

SENSITOMETRY Filed Feb. 21, 1941 HMP.

nnentor Judd 0. Baer 10 20 3o 40 do 6o 7a ao 9'01'00 .D/s'mwczz- PERCENTBH (Ittomeg Patented Oct. 31, 1944 SENSITOMETRY `ludd O. Baker, MedfordLakes, N. J., assignor to Radio Corporation of America, a corporation ofDelaware Application February 21, 1941, Serial No. 379,982

2 Claims.

This invention relates to an apparatus for the sensitometry ofphotographic emulsions. Although the apparatus is a variety ofdensitometer, it differs from the .customary densitometers in that itdoes not measure the density of a specic area or the average density ofan extended area, but instead plots a curve preferably corresponding tothe H and D curve and showing the gradation of density or transmissionover a predetermined distance in the exposed lm. In addition, theapparatus applies the curve of density to the fluorescent screen of acathode ray tube in such a manner that the film speed may be readdirectly on the face of the tube or the curve may be photographed.

In the performance of the invention, a predetermined length of'the filmof which the speed is to be ascertained is given an exposure whichvaries in a predetermined manner from one end of the strip to the other.This exposure may vary either linearly or logarithmically, but itisnecessary that the exposure variation be according to a predeterminedcurve or formula so that a uniform progression of exposure from one' endto the other of the predetermined length is secured. Such exposure maybe accomplished by the sensitometer described and claimed n myapplication Serial No. 372,815, filed January 2, 1941, -particularly theform of sersitometer shown in Figs. 7, 8 and 9 of the said application.The exposure of the iilm, of course, may be accomplshed in any otherdesired manner, as, for example, the

' exposure thereof through a variable density strip of appropriatedimensions and densities. After the test strip is developed, it isscanned photoelectrically from one end to the other and the potentialproduced by the' scanning is applied to a cathode ray tube so as` todeflect the cathode ray beam in one sense according to 'the lighttransmitted by the film, while the beam is simultaneously beingdefiected in the 'perpendicular sense in accordance with the position ofthe scanning light on the test strip. This causes the spot of light onthe'screen of the cathode ray tube to plot a curve of light transmissionagainst the length of the test strip and, if the exposure of the filmfollows a logarithmic law,- then this curve` plotted will accuratelycorrespond with 'the H and D curve which is usually plotted manually'from a4 series or readings of different individual exposures. v

It will be apparent that a single scanning of the test strip willproduce a single passage of the spot of light on the screen of thecathode ray tube and this is suificient for a photographic a' reading ofthe following specification and an recording of the characteristiccurve. However, it is not usually necessary to make a photographicrecord of the characteristic curve particularly as between differentbatches of the same type of emulsion and a visual inspection of thecurve is suflicient to determine the speed of the particular emulsion.An apparatus is therefore provided which repeats the scanning operationat suiiiciently short intervals so as to maintain the l curve apparentlycontinuously luminous so that it may be inspected.

One object of the invention is to provide an improved apparatus fordetermining thesensi- `tometric curve .of photographic emulsions.

Another object of the invention is to provide a p densitometer whichwill plot a density curve rather than ymerely measuring local or averagedensities.v

Another object of the invention is to provide anv improved type ofscanning densitometer.

lAnother object of the .invention is to provide a sensitometric devicewhich will directly produce a sensitometric curve.

Other and incidental objects of the invention will be apparent to thoseskilled in the art from inspection of the accompanying drawing in whichFigure l is a schematic diagram showing the preferred form of theinvention, and

Figure 2 is an elevation of a type of sensitometric curve such as wouldbe produced on the fluorescent screen of the cathode ray tube shown inFig. 1.

In the apparatus shown in Fig. 1, the exposed and developed film sampleis indicated at I 0. This sample or test strip is secured to the drum Ilin such a manner'that one edge thereof overhangs the end of the drum.The film may be secured to the drum in any appropriate manner; in thedrawing it is shown as merely having its two ends of the optical systemduring its passage therethrough. Light from an exciter lamp I3is'focused by an appropriate condenser lens I4 upon an aperture plat@ l5and the image of the aperture is then focused upon the film by the`objective I6 in the manner customary in the reproducton of sound fromphotographic sound records in order to secure substantially a point orlinear scanning spot. This arrangement is prefererd for the reasons thatit is not only highly satisfactory but readily available commercially,but any equivalent apparatus may be substituted for causing light topass through a small area of the film, many such devices being showninthe patented art. The light which passes through the lm strikes thephotocell I2. The current through the photocell I2 which varies inaccordance with the amount of light striking it is applied to theamplifier I1, which is preferably a D. C. type of voltage amplifier, andthe output of this amplifier is applied to the deilecting plates 22 ofthe cathode ray tube I8. This cathode ray tube is preferably of the typecommercially known as an I-conoscope and is provided with an electrongun 23 which directs a rather sharply focused beam of cathode raysthrough the pairs of deflecting plates 2l and 22 to the fluorescentscreen of the large end of the tube.

. On the same shaft with the drum II there is located a movable contact21 which is adapted to move over the potentiometer resistor 2G.Appropriate potential is applied across the resisto-r .20 by a source ofcurrent I9 and the potential derived between the midpoint of theresistor 20 and the movable contactor 21 is applied to the pair ofdeiiector plates 2l of the Iconoscope. The plates 2| produce deflectionof the cathode ray beam perpendicular to the deflection produced by theplates 22 controlled from the photocell circuit. The blank space betweenthe two ends of the potentiometer resistor 2ll is so located in relationto the co-ntact 21 that the contact arm 21 will be on this space at thesame time that the two ends of the film strip I are passing through thescanning beam and any effect of the ends of the film strip on thescanning beam is therefore thro-wn ott the curve being traced by theapparatus and, although visible on the screen of.

the tube, will not interfere with the visibility or legibilty of thecurve being traced.

In the operation of the device, after the film -sample is installed inthe drum |I the shaft 28 is caused to rotate by an appropriate drivingmeans 29 so that the drum I I, the film strip lil, and the contacter 21rotate together. This rotation should preferably be at a speed such thatthe persistence of vision plus the persistence of the image on thescreen of the cathode ray tube will cause the curve traced on thefluorescent screen to appear as a continuous -line 24. An appropriatespeed of revolution is something of the order of 1,00 R. P. M., orslightly upwards of 16 revolutions per second. Any higher sneed formlyin light transmission, the resulting plot would, of course, be astraight line, but practically, due to the variation in density with exyposure in accordance with the well-known H and D curve, the curve tracedwill depart from a straight line. shown, the curve 24 corresponds moreor less with the curve which might be produced by what is referred to insound recording work as the overall transmission. A sensitometricexposure is made on the negative film as described above, and, afterdevelopment, this is printed onto a. piece of positive lm stock which'is then developed and tested in the apparatus. In this case, the curveproduced should be as nearly a straight line as possible in order tosecure optimum sound reproduction.

If it is desired to measure emulsion speed directly, the emulsion may begiven an exposure which is varied logarithmically and then developed andscanned in the apparatus. The curve produced under such circumstanceswill be the H and D curve for that particular emulsion.

It will be apparent to those skilled in the art that my invention is notlimited to the speciiic means shown, but that the mechanism forperforming the invention shown is the preferred form in view of the factthat it can be constructed from commercially available parts and can beeasiI-y calibrated. The apparatus may, if desired, be quite considerablymodilied within the scope of the invention. For example, the photocelland exciter lamp might be interchanged without any modification of theoperation of the apparatus or the optical system might be rotated whilethe iilm remains stationary. The film might be used fiat and scanned byan appropriate optical system, such as one of the various scanningdevices of rotation may, of course. be used. if desired,

but, at too high a speed of rotation, the wear uponlthe potentiometermight become excessive, or centrifugal force might tend to throw thefilm sample I0 outwardly and thereby produce erratic readings. l

It will be apparent that the deflecting plates 22 will cause the spot ofliebt on the end of the cathode rav tube to be deflected vertically inaccordance with the light transmission through the lm sample. while. atthe same time. the defiantinef plates 2| cause the deflection thereoflaterally in accordance with the portion ,of the .film sample beinescanned at tbe corresponding instant. If the iilm sample variedperfectly uniknown in the art. Instead of the film being itself scanned.an image thereof may be produced which, in turn, would be scanned, as,for' example, by the use of a kinescope instead of the mechanicalscanning device shown. Likewise, an electrical sweep circuit controldevice might be substituted for the potentiometer 20, 21. such sweepcircuits being well known in the oscillograph art. If, as justindicated, a kinescope were used for scanning the image of the teststrip and a sweep circuit oscillator were substituted for thenotentiometer, these two circuits could be interlocked .I

in the manner customary in television scanning circuits and known inthat art instead of using a mechanical connection between the scanningand sweep circuit devices. Likewise, av eouiva- `lent electricallycontrolled curve plotting device 1.4 Sensitometric apparatus comprisinga drum shaped rotatably mounted carrier for a sensitometric strip, meansfor rotating said carrier about its axis, means for mounting saidstripon said carrier with an edge portion thereof over-v hanging theedge of said drum, a light source on one side of said strip, a photocellon the opposite In the particularv arrangement side of saidstrip, saidcarrier rotating means including` a shaft attached at one end to thebase of said carrier drum and having at its other end a contact arm, anannular resistor over which said contact arm is movable, a source ofelectrical 5 current connected to the ends of said resistor, and acathode ray tube having two pairs of plates, one pair of. plates beingconnected to said photo- J UDD O. BAKER.

